The present invention relates to power amplifiers particularly adapted for use in sound reproduction provided with means to limit the output current delivered to a load, and particularly to power amplifiers which include a driver for the power output stage of the amplifier which limits the excitation of the output stage responsive to the impedance of the load.
The present invention is an improvement on devices to protect the amplifier against faults in the load of the amplifier, such as a loudspeaker which presents an impedance to the amplifier below the design impedance of the amplifier such that the amplifier will be overloaded when driven to full output. A number of devices exist in the prior art for this purpose, but all of the devices of the prior art possess shortcomings.
One approach to preventing overload of the output devices of a power amplifier has been to incorporate a relay in the circuit which will limit the power dissipation of the output devices responsive to some current or voltage level. The U.S. Pat. No. 4,379,997 of Minora Ooishi et. al. entitled Power Amplifier discloses an amplifier provided with an electronic switch which is responsive to a muting signal to switch the voltage source for the driver to one of two power sources. U.S. Pat. No. 4,173,740 of Yuuichi Nagata et. al. entitled Speaker Protection Circuit is another example of the use of a detection circuit to actuate a relay. Such devices have the disadvantage of interrupting the output of the amplifier for a significant period of time before it can be restored to operating condition.
The Handbook for Sound Engineers, Howard W. Sams & Company 1987, at page 542, describes an audio amplifier in which the current in the emitter circuits of the output devices is monitored to measure the power dissipation of the output devices, and the current excitation to the bases of the output devices is limited if the power dissipation exceeds a particular level to protect the circuit. Circuits of this type in the prior art generally limit the effective dynamic range of the amplifier. In addition, such circuits often create distortion in the audio output from the output devices.
U.S. Pat. No. 4,494,164 of Stan L. Noel entitled Overload Protection Device discloses a similar device for lowering or controlling the excitation to a drive stage of an amplifier, but achieves this function electronically without using a mechanical relay. U.S. Pat. No. 4,789,842 of Jiri Naxera entitled Composite Transistor Device With Over-Current Protection discloses a transistor switch which is responsive to the magnitude of the input signal to clamp the base-emitter of the exciter transistor.
Another approach to protecting the output device of a power amplifier from a load of too low impedance is disclosed in U.S. Pat. No. 4,816,963 of Gary M. Eden entitled Apparatus for Protecting a Transistor in the Event of a Shorted Load Condition which compares a feedback voltage from the load with a reference voltage, and responsive to an overload condition, deactivates the amplifier periodically for short periods of time until the condition is corrected.
U.S. Pat. No. 4,321,554 to Quilter entitled Time-Delayed, Variable Output Current Limiting Means for Power Amplifiers discloses an amplifier with power output devices driven by an operational amplifier. The current output of the operational amplifier is determined by the charge on a capacitor, and that charge is in part determined by a feedback signal from the load to the capacitor which charges the capacitor in direct relation to the impedance of the load.
It is an object of the present invention to avoid the use of disabling relays or electronic switches in order to reduce the output current of the power output devices of a power amplifier responsive to the impedance of the load, and to reduce the output current of the power output devices to a greater extent than is possible with those devices of the prior art which do not use disabling relays or switches.
It is a further object of the present invention to provide a power amplifier with an input stage, an output stage adapted to be coupled to a load, and a transistor coupling stage coupled between the input stage and the output stage, including means responsive to the impedance of the load to reduce the output of both the input stage and the coupling stage when the impedance of the load is below a threshold value.